In the case of heavy machines, due to the broad scope of tasks performed by an agricultural or communal commercial vehicle, a transmission must implement highly diverse driving ranges, for example for field working activities or for travel on the road for purposes of transport or transfer, and so a correspondingly large spread between the slowest and the fastest gear step is necessary. Furthermore, heavy machines require small geometric step changes between the individual gears, which, in combination with the large spread, means that a large number of gear steps must be provided. This large number can be achieved with an acceptable degree of complexity by means of a group design of a transmission. Therefore, a heavy-machine transmission usually comprises an upstream or downstream splitter gearing part, followed by a stepped or main gearing part, a downstream group gearing part, and a reversing gearing part. A gear sequence of the transmission is predetermined via the main gearing part and is compressed by means of the upstream or downstream splitter gearing part in that each of the gear steps of the main gearing part is split by small step changes of the splitter gearing part and, therefore, the total number of gears is increased in that this total number is thus multiplied by the number of possible steps of the splitter gearing part. The gear sequence is expanded, however, by means of a downstream group gearing part in that the gear steps of the main gearing part are converted into different transmission ranges by means of large transmission ratio increments of the group gearing part. It is then possible to reverse the direction of rotation by means of the reversing gearing part, which is also usually provided, and, in combination with the other transmission parts, it is usually possible to also shift a plurality of reverse gears.
In modern heavy-machine transmissions, however, the splitter gearing part and the main gearing part are often combined to form a common splitter group part, downstream of which a group gearing part in the form of a range group part is installed. These transmissions are usually also power-shiftable, at least in part, thereby ensuring that a gear shift can be implemented without interruption of tractive force during operation of the heavy machine. In this context, heavy-machine transmissions are also often designed as so-called parallel shift transmissions, in which the power-shiftability is achieved by switching between two transmission branches according to an actuation of associated power-shift elements.
Document DE 10 2007 000 595 A1 describes a parallel shift transmission comprising a splitter group part and a range group part. Therein, the splitter group part performs the function of a main gearing part and a splitter gearing part, while the downstream range group part functions as a group gearing part. The splitter group part and the range group part are then split into two parallel transmission branches, wherein the particular transmission branch is selected by actuating an associated power-shift element. The two power-shift elements of the transmission branches are combined in a double clutch, which transfers rotational movement of a drive shaft of the parallel shift transmission to one of two input shafts of the splitter group part. Proceeding from the input shaft that has been selected, this rotational movement is transferred to a parallel-extending countershaft of the two transmission branches at a stepped transmission ratio according to the selection of one a plurality of gear steps of the splitter group part. Proceeding therefrom, the rotational movement of the respective countershaft is transferred to an output shaft of the parallel shift transmission at a selected range transmission ratio, which is defined by shifting to an associated group step of the range group part. The gear steps of the splitter group part are distributed to the two transmission branches in an alternating manner in the sequence of their associated stepped transmission ratios, and thus a switch back and forth between the two transmission branches occurs as the individual stepped transmission ratios are shifted in succession. Therefore, it is possible to preselect the associated gear step in the currently unloaded transmission branch before shifting to the next stepped transmission ratio, thereby ensuring that it is only necessary to disengage the power-shift element of one branch and engage that of the other branch in order to implement the shift. As a result, the stepped transmission ratio can be changed under load and, therefore, without interruption of tractive force. The group steps of the downstream range group part, however, can each be implemented by means of both transmission branches, wherein a transition from the last gear step in a group step to the first gear step of the subsequent group step can be carried out under load in this case as well. Overall, a fully power-shiftable parallel shift transmission is thereby formed, in which no more than two gearwheel engagements ever occur under load at any speed, and which is therefore characterized by a high level of efficiency.